Since the 1970's, two categories of data transmission networks have become progressively more differentiated:
long-distance networks, based on the utilization of telephonic transmission media (for example transmission lines of the P.T.T. public utility in France), PA1 local networks, limited to a private domain (building, factory, campus, hospital), which are not dependent upon the characteristics of telephonic transmission lines. PA1 a DTE, as a data transmitter, is either silent, or in the course of data transmission. PA1 a DTE, when silent, can, as a receiver, either detect silence, or receive data coming from one or more other DTEs. If the data received are coming from more than one DTE at the same time, the value of this data is without significance, as is the case when in the presence of a collision. PA1 a DTE which is in the course of transmission, can, as a receiver, either detect that its transmission is proceeding normally (without collision with the data transmitted by one or more other DTEs), or that a collision is occurring. PA1 (a) Each DTE which attempts to transmit data determines, before beginning, that it is receiving only silence, which must endure for at least a certain time (termed the interframe delay). PA1 (b) In the absence of collision, transmission is maintained for at least a certain duration; each set of data transmitted, termed a frame, must have a minimum length determined as a function of the duration of its propagation in the maximum standard configuration of the network. PA1 (c) After detection- of a collision, there is transmission of interference data, followed by a return to silence; this means that if a DTE detects a collision, it continues to transmit for a certain time, termed the jamming duration, then ceases to transmit. This duration is such that a frame suffering a collision is shorter than the duration defined in b above. PA1 (d) Upon reception, all data disPlaying an insufficient length is rejected (length less than the minimum length defined in b above). PA1 (e) In the event of interruption of a transmission, due to a collision, there is a repetition of the attempted transmission after a brief delay which increases from one attempt to the next, for a given frame, in a way which is sufficient that a high rate of collisions is not very probable.
Whether it belongs to the first categorY or the second category of transmission networks defined above, any network is constituted by a number of units, generally designated as data terminal equipment, abbreviated DTE, or in common parlance terminals or stations, These DTEs communicate with one another through the intermediarY of a transmission sYstem. This system may have a bus architecture, for example and can be constituted by a pair of telephone wires or a coaxial cable.
In local networks, data transmission is effected at higher rates than over long distance networks, for example 100 kbit/s to 100 Mbit/s, for distances between DTEs of from a few hundred meters to a few kilometers. (For long-distance networks, the DTE rates are generally less than 100 kbit/s.)
Because the DIEs of a local network with bus architecture share a single-transmission sYstem (transmission line), it is important that only one DTE at a time transmits onto the same transmission line. To resolve this problem, the DTEs are generally equipped with decision mechanisms which authorize the sending of messages only on condition that the line is free. For this, according to a preferred method, each terminal DTE continuously monitors the signals present on the line, and in the event that a message is present on the line, no other message is authorized to he sent. In this way the access methods conceived for local bus networks seek to limit conflicts of access due to simultaneous sending by more than one station (collisions). This process is familiar under the acronym CSMA.
However, despite this precaution, it can result that a station fails to detect the presence of a message on the transmission line at the instant it itself commands the sending of a message, although another station is in fact transmitting. In this type of network, this situation, termed "collision," is impossible to avoid entirely, notably because of the delay of propagation of signals on the line. To remedy this difficulty, generally a device is used termed a collision detector, which interrupts transmission of a message in progress when such a situation is detected. The detection of a collision causes cancellation of the transmIssion, following whIch a new attempt is made to transmit, with transmission taking place only if the transmission line has become free. This method is known under the acronym CSMA/CD (carrier sense multiple access with collision detection).
Local networks of the CSMA/CD type have been standardized by the IEEE committee of the Institute of Electrical and Electronic Engineers, in the form of a standard termed 802.3, (adopted by the ISO, International Standardization Organization. as the standard ISO 8802.3), whose essential provisions are the following:
The principles of the CSMA/CD access method are the following:
The IEEE standard 802.3 elegantly defines the format of frames. The frame is an individual message. It is structured, and includes a starting and ending signal, synchronization signals, the address of the statIon to which the message is intended to be sent, the address of the sending station, the length of data, useful data, etc... . In other words, the frame is the elementary block of information transmitted by any station, which passes over the transmission line.
The standard 802.3 additionally defines the protocol for the dialog between the stations. This protocol defines the rules of access to different stations, and thus constitutes a sYstem which regulates the communication among them, without organizing them hierarchically.
Thus, for a local network of the type of ETHERNET, with a transmission rate of 10 Mbit/s, the interframe duration is 9.6 microseconds (which corresponds to the duration of 96 bits), and the jam duration is 3.3 microseconds (which corresponds to the duration of 33 bits). The minimum length of the frame without collision is 64 octets (512 bits), and the maximum length is 1500 octets (12,000 bits).
The physical configuration of a local network of the type of ETHERNET is the following:
The standard transmission medium is a coaxial cable with characteristic impedance of 50 ohms, equipped at each end with a 50 ohm coupler. Each DTE is connected to the coaxial cable through a medium access unit, commonly termed a transceiver.
The basic network is constituted by a segment of cable along which access unit connectors are distributed.
More generally, a local network is formed by a set of a number of basic networks termed elementary segments, between which repeaters are installed. These retransmit onto one cable the signals theY receive on another cable, while restoring their initial signal quality. The functioning of a network with a number of segments is ensured if the network is constituted by one principal segment, and secondary segments, with each secondary segment connected to the principal segment either by a repeater or by a pair of repeaters connnected to one another. The repeaters and transceivers include collision detectors.
When a local network of the type of ETHERNET is first installed in the various buildings of a private facility, it is desirable to verify its functioning before the various stations are connected, by means of the access units, to the various segments of the network. This process is termed the acceptance of the network.
To do this, in general two testers are used, which are mobile stations, which can function in the same manner as the stations normally connected to the various segments of the network, and which in addition can create conditions of abnormal operation, in order to see whether these are detected at the various specific locations of the network bY particular components of it.
Testers are known whIch make it possible to verify proper functioning of a local network and in particular to verify whether detections of collisions are detected normally. One of these is the E-20 tester of the French company EXPERDATA, located at 14 rue de Silly, 92100 Boulogne.
This tester includes a generator of packets of frames which, in a continuous fashion, produces packets of small, medium and large size. The interframe delay can be caused to vary, and network traffic loads varying from 2% to the maximum rate of 99.2% can be obtained. (The maximum load, also called the maximum flow rate, is that which is obtained when a series of frames is sent separated from one another by the interframe delay of 9.6 microseconds defined above.) This tester can either transmit normally, respecting the CSMA/CD protocol, or function according to a collision mode, so as to deliberately provoke collisions; thus, in this case, when a collision detector fails to indicate a collision, it is considered to be defective.
The drawback of testers of the E-20 tYpe is their bulk, and the fact that they are made up of small discrete logic components. Moreover, they do not integrally implement the above-mentioned standard 802.3, notably in that they do not perform the standard Backoff algorithm (repetition of transmission of a frame, a predetermined number of times, when there is a collision).